A return to model building and flying. Part 37

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March 2007

Wildcat Revisited

Recently taking advantage of a break in the weather and finding the only dry
spot in the CL circle where I didn't sink into the mud, of course it was up
wind, I tried out some modifications to the Wildcat.

One of the things I did earlier was to rake the undercarriage forward as
much as the structure and ironmongery would permit. This was an attempt to cope
with our potholed uneven grass circle, and has been a success. The model has
now lost it's tendency to want to flip over on landing every time, which in
turn makes it a bit more tolerant on landing approach angles. This may work
against me when I finally managed to get the carrier deck up and working, but
for now it's working and I can practice. At present the ground is so
waterlogged that it doesn't take much of an imagination to think I am flying
from a carrier over the sea.

The second modification came about after my SC.15 died a death; it needs a
new piston liner, But faced with the usual dilemma of, is it cheaper to repair
or acquire a new engine? I decided to not repair it, at least for the present.
As my Thunder Tiger.15 had ripped my Hallam Swift around the sky at a great
rate of knots, I decided to refit the carb and shoehorn it in as a replacement
for the SC. The result was a great dissapointment, the SC.15 had obviously
developed a great deal more power. The only thing I can attribute this striking
difference to, is the TT carb. In the Swift I was running the engine with a
plain venturi which had a bigger cross sectional area, but modifying the carb
would not have been as simple as boring out a venturi.

My AP.15 would not fit as the rear mounted pancake silencer would foul the
tank. The only other option was to fit a .19, but my two recently purchased
ASP.21's (the nearest thing I have to a .19) are earmarked for a Tigercat
twin.

Ever since John
Walton (Hi John) had shown me an MDS.18 in his possesion, I had wanted one. It
has the not unique, but unusual design feature, ala MVVS, of being able to
rotate the cylinder to make it into a rear exhaust setup. It also has a raked
back needle assembly like more recent Thunder Tiger engines, plus the fact that
I like MDS engines. Deciding that this might be a good replacement for the now
deceased SC, I found a nearly new one on eBay at the right price.

I have to say that with the engine being a lot heavier than the SC.15, I was
not expecting much difference in the performance. This looked like it was
probably true when it trundled through the wet grass trying to takeoff; it was
not exactly leaping into the air. The SC would have been off in half the
distance.

Once airborne things took on a different feel, and the top speed was
definitely higher. But the real eye opener was the slow flight. The
controllability of the carb was quite astonishing, with no dead spots or sudden
transitions, and there is obviously enough power available to hover the
Wildcat. I was able to get the model into a vertical position almost at will,
and still have enough grunt to open up a rise vertically, even managing to
touch the tail on the floor at one point and then rise up and resume level
flight. Not with the confidence and aplomb that Jan Odeyn can achieve, but I am
getting there slowly.

It's not often that I leave the field with a glow of satisfaction, but this
was definitely one of them.

Jan Odeyn showing what can be done with a Wildcat and an O.S.15.

The only modification I made to the carb, was to cut the internal throttle
barrel spring in half to try and reduce it's tension. If you have ever
dismantled an RC carb, you will know that most have a spring that biases the
throttle barrel outwards, the purpose of which I am guessing is to stop any end
float that would affect the midrange needle setting. The only problem is, in
doing so it presses the barrel against the retaining screw, and as the barrel
has a helical groove to facilitate the movement of the midrange needle in and
out of the spraybar, this tends to bias the throttle to full open; a problem
that has caused me headaches for other reasons.

Most of these springs are needlessly strong. No problem in an RC model being
moved by an actuator, but any extraneous force applied to a CL multi bellcrank
system that relies on line tension, can be. Whether this modification accounts
for the satisfyingly pleasant throttle responce, I won't know until I can try
it on other engines.

Crusader Flight Trial

Ever had one of those mornings when things don't go right from the moment
you wake up? Well this was one of them. I had a dull headache from the word go,
but it was the first reasonable weather we had had for some time, it wasn't
blowing a gale or pouring down with rain, so I threw evering in the car and
drove to the field.

It was breezy but not unflyable. Problem number one was I could not adjust
the midrange jet on the carb as I had not got a jewellers screwdriver narrow
enough to fit in the deeply recessed screw hole. Next was the throttle stop
screw jammed the throttle every time I tried to set the correct aperture for a
tick over. Oh well, At least I could see if the thing flew even if it was flat
out.

I had decided the night before to use the arrestor hook as an anchor for my
remote release, not thinking things through. The model of course, has a
tricycle undercarriage, and the hook is high up in it's retracted position.
When the engine ran it pulled the nose down and released the hook. The take off
was a series of ploughing sessions, the prop hurling sods of turf unto the air
as the Leo.28 kept struggling manfully to pull the model through a the sodden
uneven ground, against the resistance of the hook ploughing a furrow round the
circle behind. Somehow the model took off with half the field still attached to
the hook, and despite my worries concerning the all moving tailplane, flew
quiet normally.

The non-adjustmet of the carb had of course, turned the throttle into an on
or off affair, trying to throttle down caused the engine to cut. There followed
a bumpy landing with the small nose wheel digging into the ground and flipping
the model over. Fortunately the landing was slow and the ground very soft, so
no damage done. Even the Wildcat was not flying in it's normal pleasant manner
when I gave it a whirl, so it was the time for discretion to be the better part
of valour, and call it a day. Our site is peculiar in that if the wind blows
from certain directions there is a lot of turbulence, combine that with a
blustery wind and things can get, interesting! to say the least This may have
been one of those days.

Back home a much bigger nose wheel was fitted, and the carb dismantled. The
problem with the carb turned out to a manufacturing one, the Leo carb is a copy
of an OS carb but with one lack of attention to detail. An OS throttle stop
screw has a reduced diameter end where it fits into the helical groove in the
throttle barrel, the Leo just had a plain screw. The result is, it is OK as
just a retaining screw, but just jams in the groove if it is used for the
purpose it is intended. Grinding the end of the screw end to a shallow taper
solved the problem.

Now all I need is some clement weather to try it again.

Well that day
arrived, modest wind but cold. The Leo carb continues to be uncooperative,
there seem to be too many basic design flaws in it. I ended up with the
midrange needle screwed in as far as it will go and it is still not enough. As
there is no physical way of moving the spraybar towards the needle to
compensate, I am a little screwed. I have other carbs with the same spigot
diameter, but all with huge choke diameters, and as my money sitution has
become extremely dire at present, life in general is has been very unkind of
late and it will take many months to get my finances strait again, so I can't
afford to buy yet another carb. This is deeply annoying as the engine is fine,
and also nice and light.

The next alternative was to change the engine, the only one I have that
remotely resembles the weight of the Leo.28, is an Enya.30 BB, which I hope
will be more cooperative in the carb department. All the others I have in my
possession and would like to use, are between 50 and 100 grams heavier, which
in turn would mean adding a considerable amount of weight to the tail.

I have always thought the fuselage was a little on the short side, probably
a hangover from the days when it was designed, when unsilenced light engines
were the order of the day, This problem crops up time and time again with older
designs, especially ones from the USA, of which the crusader is one example,
the Brodak Buster, which I will probably be building next, is another.

I also flew the Wildcat again and proved that the engine throttling was no
fluke the first time I used the MDS.18. In a word it's 'wonderful'. Makes me
want to cuddle and stroke the engine just to hear it purr. Does anyone have any
idea what has happened to MDS engines?

The next time I flew the Crusader with the Enya 30 BB I ran into similar
problems with the Enya carb that I had with the Leo, it would just jam in the
3/4 closed position. This was getting frustrating. As there was not much I
could do at the field I dismantled the carb at home.

I was surprised to find a design/build flaw in the Enya carb, I would have
thought Enya would have been above dropping such a clanger. There where
actually three faults. The first being, the barrel retention screw; it was too
long. Screwed in as far as it would go (as it should be) it jammed in the
barrel groove at certain positions. Next was the throttle stop screw, there was
no way it could work as it was supposed to. There is a flat ground edge of the
barrel on it's inner end. This was ground at the wrong angle, enabling the
adjustment screw to lock the whole movement of the throttle. Lastly, as usual,
the bias spring was too strong and the barrel would tighten up at certain
positions when rotated. This is just not good enough

Cutting the spring in half, refiling the throttle stop flat at a slightly
different angle, and shortening the retention screw, has improved things a lot.
Things move and stop as they are supposed to, and the closed position can be
adjusted now.

I find it ironic that least troublesome carbs I have used have been the
cheapest, those from Russia and China (For those that don't know, Leo is
Taiwanese, as is Thunder Tiger)

This throttle problem is now beginning to p**s me off. The Enya mod's did
make a difference but the throttle responce was awful, with a long lag between
opening the throttle and anything happening, followed by an equally long
responce going the other way. Clearly unusable. In desperation I replaced the
Enya with my MDS.28 which I know works. Did it hell in this model! Yes it did
throttle better, but it was far from ideal.

Now I was forced to sit and examine things very closely and carefully. The
not quite so obvious thing was that the total throw available in the throttle
pushrod was slightly shorter than the throttle arm has to move. There is also a
in increasing resistance as the throttle is closed. This was helped by cutting
the barrel spring in half. but is still quite noticeable. It seemed to be not
too serious on the ground, but maybe it was in flight. Apart from digging holes
in the wing to get at the bellcranks, the only solution I could come up with
was to make a lever mechanism that would amplify the movement, and remove the
Enya throttle barrel spring entirely.

The full throw was achieved by the lower arm being 50% longer than the top
one

It's becoming obvious that I have to pay much more attention to the whole
bellcrank system to make sure there is enough movement and travel in future,
especially it's all mounted inside the wing. I'm on another steep learning
curve at present.

On the bench this all seems to work but the flight test report will have to
wait until next month.